Antibody-based protection against HIV infection by vectored immunoprophylaxis

Abstract

Despite tremendous efforts, development of an effective vaccine against human immunodeficiency virus (HIV) has proved an elusive goal. Recently, however, numerous antibodies have been identified that are capable of neutralizing most circulating HIV strains. These antibodies all exhibit an unusually high level of somatic mutation, presumably owing to extensive affinity maturation over the course of continuous exposure to an evolving antigen. Although substantial effort has focused on the design of immunogens capable of eliciting antibodies de novo that would target similar epitopes, it remains uncertain whether a conventional vaccine will be able to elicit analogues of the existing broadly neutralizing antibodies. As an alternative to immunization, vector-mediated gene transfer could be used to engineer secretion of the existing broadly neutralizing antibodies into the circulation. Here we describe a practical implementation of this approach, which we call vectored immunoprophylaxis (VIP), which in mice induces lifelong expression of these monoclonal antibodies at high concentrations from a single intramuscular injection. This is achieved using a specialized adeno-associated virus vector optimized for the production of full-length antibody from muscle tissue. We show that humanized mice receiving VIP appear to be fully protected from HIV infection, even when challenged intravenously with very high doses of replication-competent virus. Our results suggest that successful translation of this approach to humans may produce effective prophylaxis against HIV.

Figure 1. VIP protects against HIV-mediated CD4 cell depletion in humanized mice

a, Xenogen imaging of a representative Rag2^−/−γc^−/− mouse 15 weeks after intramuscular injection of 1×10¹⁰ genome copies (GC) of AAV2/8 expressing luciferase. b, Quantitation of human IgG by ELISA following intramuscular injection of 1×10¹¹ GC of the optimized expression vector producing b12-IgG in either immunodeficient NOD/SCID/γc^−/−(NSG) and Rag2^−/−γc^−/− (Rag2) or immunocompetent c57BL/6 (B6) and Balb/C mice (plot shows mean and standard error, n=4). c, Concentration of human IgG in circulation as measured by total human IgG ELISA on serum samples taken 6 weeks after intramuscular injection of vector expressing either luciferase or b12-IgG (N.D. = not detected). d, Depletion of CD4 T-cells in humanized mice following intraperitoneal (IP) challenge with 10ng p24 NL4-3 into animals that received AAV2/8 vectors expressing luciferase (left) or b12-IgG1 (right) 6 weeks earlier (n=6).

Figure 2. Comparison of protection mediated by various broadly neutralizing HIV antibodies

a, Concentration of antibody in circulation as measured by total human IgG ELISA on serum samples taken after intramuscular injection of vectors expressing four broadly neutralizing HIV antibodies (n=8). b, Comparison of the relative effectiveness of four broadly neutralizing HIV antibodies in protecting HuPBMC-NSG humanized mice against CD4 cell depletion following intravenous HIV challenge with 5ng p24 NL4-3 (n=8). c, HIV p24 detection by immunohistochemical (IHC) staining of sections taken from spleens 8 weeks post-challenge. Arrows indicate cells scored as positive for p24 expression. Scale bar represents 40 micrometers. d, Quantitation of IHC staining of spleen denoting the relative frequency of p24 expressing cells in spleens of infected animals. Asterisks indicate outcomes significantly different than luciferase control mice versus mice expressing antibodies by two-tailed t test (n=4–6, N.D.= Not Detected) **P<0.01, ***P<0.0001. Plots a and b show mean and standard error, Plot d shows mean and s.d.

Figure 3. Robustness of CD4 cell protection mediated by b12 antibody

CD4 cell depletion in HuPBMC-NSG humanized mice as a result of intravenous challenge with the dose of NL4-3 indicated on the far right. Mice expressing luciferase (left plots) were susceptible to CD4 cell loss whereas those expressing b12 (right plots) demonstrated protection from HIV at all doses (n=8).

Figure 4. Determination of the minimum protective dose of VRC01 in vivo

(left) VRC01 expression over time as a function of dose as determined by total human IgG ELISA on serum samples taken following AAV administration (n=8). Mice receiving luciferase-expressing vector exhibited no detectable human antibodies (n=12). (middle) Concentration of VRC01 in serum one day prior to challenge, 3 weeks after adoptive transfer of human PBMCs and 15 weeks after intramuscular administration of the indicated dose of AAV as determined by a gp120-specific ELISA to measure the fraction of antibodies capable of binding HIV (n=8–12). (right) CD4 cell depletion in HuPBMC-NSG humanized mice as a result of intravenous challenge with 10ng of NL4-3 into animals expressing a range of VRC01, demonstrating the minimum dose of antibody necessary to protect against infection. Left and right plot show mean and standard error, middle plot shows individual animals and mean (n=8–12).